It’s an epidemic now. Behind the growing number of cases: Mucus membranes that produce a key component of tears naturally dry up as people age. Spending a lot of time in low-humidity environments, such as air-conditioned office buildings or airplane cabins, also can promote dry eye, as a recent study published in the journal Cornea found. And staring at a computer or smartphone screen for long periods also can contribute to dry eye, as people forget to blink.